method DOMMatrix.multiplySelf

The multiplySelf() method of the DOMMatrix interface multiplies a matrix by the otherMatrix parameter, computing the dot product of the original matrix and the specified matrix: A⋅B. If no matrix is specified as the multiplier, the matrix is multiplied by a matrix in which every element is 0 except the bottom-right corner and the element immediately above and to its left: m33 and m34. These have the default value of 1.

MDN Reference